Plating bath and method

ABSTRACT

Tin-containing electroplating baths having a combination of certain brightening agents provide tin-containing solder deposits having reduced void formation and smooth morphology.

The present invention relates generally to the field of electrolyticmetal plating. In particular, the present invention relates to the fieldof electrolytic tin plating.

Metals and metal-alloys are commercially important, particularly in theelectronics industry where they are often used as electrical contacts,final finishes and solders. The use of tin-lead, once the most commontin-alloy solder, has declined due to the increasing restrictions onlead. Lead-free solders, such as tin, tin-silver, tin-copper,tin-bismuth, tin-silver-copper, and others, are common replacements fortin-lead solders. These solders are often deposited on a substrate usinga plating bath, such as an electroplating bath.

Methods for electroplating articles with metal coatings generallyinvolve passing a current between two electrodes in a plating solutionwhere one of the electrodes (typically the cathode) is the article to beplated. A typical tin plating solution comprises dissolved tin ions,water, an acid electrolyte such as methanesulfonic acid in an amountsufficient to impart conductivity to the bath, an antioxidant, andproprietary additives to improve the uniformity of the plating and thequality of the metal deposit. Such additives include surfactants andgrain refiners, among others.

Certain applications for lead-free solder plating present challenges inthe electronics industry. For example, when used as a capping layer oncopper pillars, a relatively small amount of lead-free solder, such astin-silver solder, is deposited on top of a copper pillar. In platingsuch small amounts of solder it is often difficult to plate a uniformheight of solder composition on top of each pillar, both within a dieand across the wafer. The use of known solder electroplating baths alsoresults in deposits having a relatively rough surface morphology, forexample, having a mean surface roughness (Ra) of about 800 nm or greateras measured by optical profilometry. Such relatively rough surfacemorphology often correlates with void formation in the solder afterreflow, which ultimately creates concerns regarding solder jointreliability. Accordingly, there is interest in the industry for a puretin or tin-alloy solder deposit which avoids the problem of a relativelyrough surface morphology, and which provides improved within dieuniformity.

Many tin electroplating baths are known. When lustrous surfaces aredesired, brighteners are typically employed in the tin electroplatingbath. Conventional brighteners include aldehydes, ketones, carboxylicacids, carboxylic acid derivatives, amines or mixtures thereof. Forexample, U.S. Pat. No. 4,582,576 discloses a tin or tin-leadelectroplating bath containing at least one primary brightening agentselected from the group of aromatic aldehydes, acetophenones, andcarbonyl compounds having the formula Ar—C(H)═C(H)—C(O)—CH₃ wherein Aris phenyl, naphthyl pyridyl, thiophenyl or furyl, and at least onesecondary brightening agent selected from the group of lower aliphaticaldehydes and substituted olefins of the formula R₄C(R₃)═C(R₂)R₁ whereinR₁ is a carboxy, carboxamide, alkali metal carboxylate, ammoniumcarboxylate, amine carboxylate or allyl carboxylate, and R₂, R₃ and R₄are each independently hydrogen or lower alkyl groups. This patentdiscloses that such primary and secondary brightening agents are eachused in relatively large amounts, that is, in tenths of grams or moreper liter. When used in certain electronic applications, suchconventional tin electroplating baths result in tin deposits havingvoids, such as when used to form a pure tin cap on a copper pillar,and/or result in tin deposits having unacceptable morphology, and/orresult in deposits having a relatively high level of impurities.Accordingly, there remains a need in the industry for electroplatingbaths and methods for depositing a tin-containing solder layer,particularly as a cap on a copper pillar, that has both acceptablemorphology and is substantially free of voids.

The present invention provides an electroplating composition comprising:a source of tin ions; an acid electrolyte; 0.0001 to 0.045 g/L of afirst grain refiner; 0.005 to 0.75 g/L of an α,β-unsaturated aliphaticcarbonyl compound as a second grain refiner; a nonionic surfactant; andwater; wherein the first grain refiner is chosen from a compound offormula (1) or (2)

wherein each R¹ is independently (C₁₋₆)alkyl, (C₁₋₆)alkoxy, hydroxy, orhalo; R² and R³ are independently chosen from H and (C₁₋₆)alkyl; R⁴ isH, OH, (C₁₋₆)alkyl or O(C₁₋₆)alkyl; m is an integer from 0 to 2; each R⁵is independently (C₁₋₆)alkyl; each R⁶ is independently chosen from H,OH, (C₁₋₆)alkyl, or O(C₁₋₆)alkyl; n is 1 or 2; and p is 0, 1 or 2.

The present invention also provides a method of depositing atin-containing layer on a semiconductor substrate comprising: providinga semiconductor wafer comprising a plurality of conductive bondingfeatures; contacting the semiconductor wafer with the compositiondescribed above; and applying sufficient current density to deposit atin-containing layer on the conductive bonding features.

Also provided by the present invention is a semiconductor devicecomprising a plurality of conductive bonding features and atin-containing layer disposed on the bonding feature wherein thetin-containing layer is substantially free of voids as plated, and afterreflow.

As used throughout this specification, the following abbreviations shallhave the following meanings, unless the context clearly indicatesotherwise: ASD=A/dm²=ampere per square decimeter; ° C.=degree Celsius;g=gram; mg=milligram; L=liter; Å=angstrom; nm=nanometer;μm=micron=micrometer; mm=millimeter; min=minute; DI=deionized; andmL=milliliter. All amounts are percent by weight (“wt %”) and all ratiosare weight ratios, unless otherwise noted. All numerical ranges areinclusive and combinable in any order, except where it is clear thatsuch numerical ranges are constrained to add up to 100%.

As used throughout this specification, the term “plating” refers tometal electroplating. “Deposition” and “plating” are usedinterchangeably throughout this specification. “Pure tin” refers to atin deposit that is not a tin-alloy, although such deposit may containup to 5 atomic % of impurities. “Halide” refers to chloride, bromide,iodide and fluoride. The articles “a”, “an” and “the” refer to thesingular and the plural. “Alkyl” refers to linear, branched and cyclicalkyl. “Aryl” refers to aromatic carbocycles and aromatic heterocycles.The term “(meth)acrylic” refers to both “acrylic” and “methacrylic”.When an element is referred to as being “disposed on” another element,it can be directly on the other element or intervening elements may bepresent therebetween. In contrast, when an element is referred to asbeing “disposed directly on” another element, there are no interveningelements present. As used herein, the term “and/or” includes any and allcombinations of one or more of the associated listed items.

Compositions of the present invention comprise: a source of tin ions; anacid electrolyte; 0.0001 to 0.045 g/L of a first grain refiner; 0.005 to0.75 g/L of an α,β-unsaturated aliphatic carbonyl compound as a secondgrain refiner; a nonionic surfactant; and water; wherein the first grainrefiner is chosen from a compound of formula (1) or (2)

wherein each R¹ is independently (C₁₋₆)alkyl, (C₁₋₆)alkoxy, hydroxy, orhalo; R² and R³ are independently chosen from H and (C₁₋₆)alkyl; R⁴ isH, OH, (C₁₋₆)alkyl or O(C₁₋₆)alkyl; m is an integer from 0 to 2; each R⁵is independently (C₁₋₆)alkyl; each R⁶ is independently chosen from H,OH, (C₁₋₆)alkyl, or O(C₁₋₆)alkyl; n is 1 or 2; and p is 0, 1 or 2.

Any bath-soluble divalent tin salt may suitably be used as the source oftin ions. Examples of such tin salts include, but are not limited to,tin oxide and salts such as tin halides, tin sulfates, tinalkanesulfonate such as tin methanesulfonate and tin sulfonate, tinarylsulfonate such as tin phenylsulfonate, tin phenolsulfonate, tincresolsulfonate, and tin toluenesulfonate, tin alkanolsulfonate, and thelike. When tin halide is used, it is preferred that the halide ischloride. It is preferred that the tin compound is tin oxide, tinsulfate, tin chloride, tin alkanesulfonate or tin arylsulfonate. Morepreferably, the tin salt is the stannous salt of methanesulfonic acid,ethanesulfonic acid, propanesulfonic acid, 2-hydroxyethane-1-sulfonicacid, 2-hydroxypropane-1-sulfonic acid, 1-hydroxypropane-2-sulfonicacid, phenylsulfonic acid, toluenesulfonic acid, phenolsulfonic acid, orcresolsulfonic acid, and even more preferably methanesulfonic acid,phenylsulfonic acid or phenolsulfonic acid. Mixtures of tin salts may beused. The tin compounds useful in the present invention are generallycommercially available from a variety of sources and may be used withoutfurther purification. Alternatively, the tin compounds useful in thepresent invention may be prepared by methods known in the literature.Typically, the amount of tin ions in the present composition is in therange of 10 to 300 g/L, preferably from 20 to 200 g/L, and morepreferably from 30 to 100 g/L.

Any acid electrolyte that is bath-soluble and does not otherwiseadversely affect the electrolyte composition may be used in the presentinvention. Suitable acid electrolytes include, but are not limited to:alkanesulfonic acids such as methanesulfonic acid, ethanesulfonic acidand propanesulfonic acid; arylsulfonic acids such as phenylsulfonicacid, toluenesulfonic acid, phenolsulfonic acid, and cresolsulfonicacid; alkanolsulfonic acids; sulfuric acid; sulfamic acid; and mineralacids such as hydrochloric acid, hydrobromic acid and fluoroboric acid.Alkanesulfonic acids and arylsulfonic acids are preferred acidelectrolytes, and alkanesulfonic acids are more preferred.Methanesulfonic acid is particularly preferred. Mixtures of acidelectrolytes are particularly useful, such as, but not limited to,mixtures of alkanesulfonic acids and sulfuric acid. Thus, more than oneacid electrolyte may be used advantageously in the present invention.The acid electrolytes useful in the present invention are well-known andare generally commercially available and may be used without furtherpurification. Alternatively, the acid electrolytes may be prepared bymethods known in the literature. Typically, the amount of acid in thepresent compositions is in the range of 10 to 1000 g/L, preferably from20 to 750 g/L, and more preferably from 30 to 500 g/L.

The present compositions comprise a mixture of a first grain refiner anda second grain refiner. The first grain refiner is chosen from acompound of formula (1) or (2)

wherein each R¹ is independently (C₁₋₆)alkyl, (C₁₋₆)alkoxy, hydroxy, orhalo; R² and R³ are independently chosen from H and (C₁₋₆)alkyl; R⁴ isH, OH, (C₁₋₆)alkyl or O(C₁₋₆)alkyl; m is an integer from 0 to 2; each R⁵is independently (C₁₋₆)alkyl; each R⁶ is independently chosen from H,OH, (C₁₋₆)alkyl, or O(C₁₋₆)alkyl; n is 1 or 2; and p is 0, 1 or 2.Preferably, each R¹ is independently (C₁₋₆)alkyl, (C₁₋₃)alkoxy, orhydroxy, and more preferably (C₁₋₄)alkyl, (C₁₋₂)alkoxy, or hydroxy. Itis preferred that R² and R³ are independently chosen from H and(C₁₋₃)alkyl, and more preferably H and methyl. Preferably, R⁴ is H, OH,(C₁₋₄)alkyl or O(C₁₋₄)alkyl, and more preferably H, OH, or (C₁₋₄)alkyl.It is preferred that R⁵ is (C₁₋₄)alkyl, and more preferably (C₁₋₃)alkyl.Each R⁶ is preferably chosen from H, OH, or (C₁₋₆)alkyl, more preferablyH, OH, or (C₁₋₃)alkyl, and yet more preferably H or OH. It is preferredthat m is 0 or 1, and more preferably m is 0. Preferably, n=1. It isreferred that p is 0 or 1, and more preferably p=0. A mixture of firstgrain refiners may be used, such as 2 different grain refiners offormula 1, 2 different grain refiners of formula 2, or a mixture of agrain refiner of formula 1 and a grain refiner of formula 2. It ispreferred that the first grain refiner is a compound of formula (I).

Exemplary compounds useful as the first grain refiner include, but arenot limited to, cinnamic acid, cinnamaldehyde, benzylidene acetone,picolinic acid, pyridinedicarboxylic acid, pyridinecarboxaldehyde,pyridinedicarboxaldehyde, or mixtures thereof. Preferred first grainrefiners include cinnamic acid, cinnamaldehyde, and benzylidene acetone.

The first grain refiner is present in the plating baths of the inventionin an amount of 0.0001 to 0.045 g/L. Preferably, the first grain refineris present in an amount of 0.0001 to 0.04 g/L, more preferably in anamount of 0.0001 to 0.035 g/L, and yet more preferably from 0.0001 to0.03 g/L. Compounds useful as the first grain refiners are generallycommercially available from a variety of sources and may be used as isor may be further purified.

The second grain refiner useful in the present plating baths is anα,β-unsaturated aliphatic carbonyl compound. Suitable α,β-unsaturatedaliphatic carbonyl compound include, but are not limited to,α,β-unsaturated carboxylic acids, α,β-unsaturated carboxylic acidesters, α,β-unsaturated amides, and α,β-unsaturated aldehydes.Preferably, the second grain refiner is chosen from α,β-unsaturatedcarboxylic acids, α,β-unsaturated carboxylic acid esters, andα,β-unsaturated aldehydes, and more preferably α,β-unsaturatedcarboxylic acids, and α,β-unsaturated aldehydes. Exemplaryα,β-unsaturated aliphatic carbonyl compounds include (meth)acrylic acid,crotonic acid, (C₁₋₆)alkyl (meth)acrylate, (meth)acrylamide, (C₁₋₆)alkylcrotonate, crotonamide, crotonaldehyde, (meth)acrolien, or mixturesthereof. Preferred α,β-unsaturated aliphatic carbonyl compounds are(meth)acrylic acid, crotonic acid, crotonaldehyde, (meth)acrylaldehydeor mixtures thereof.

The second grain refiner is present in the plating baths of theinvention in an amount of 0.005 to 0.75 g/L. Preferably, the secondgrain refiner is present in an amount of 0.005 to 0.5 g/L, morepreferably in an amount of 0.005 to 0.25 g/L, and yet more preferablyfrom 0.01 to 0.25 g/L. Compounds useful as the second grain refiners aregenerally commercially available from a variety of sources and may beused as is or may be further purified.

One or more nonionic surfactants are used in the present compositions.Typically, the nonionic surfactants have an average molecular weight offrom 200 to 100,000, preferably from 500 to 50,000, more preferably from500 to 25,000, and yet more preferably from 750 to 15,000. Such nonionicsurfactants are typically present in the electrolyte compositions in aconcentration of from 1 to 10,000 ppm, based on the weight of thecomposition, and preferably from 5 to 10,000 ppm. Preferred alkyleneoxide compounds include polyalkylene glycols, including ethyleneoxide/propylene oxide (“EO/PO”) copolymers alkylene oxide condensationproducts of an organic compound having at least one hydroxy group and 20carbon atoms or less, and tetrafunctional polyethers derived from theaddition of different alkylene oxides to ethylenediamine compoundsprepared by adding oxypropylene to polyoxyethylene glycol.

Typically, the polyalkylene glycols useful in the compositions are thosehaving an average molecular weight in the range of 200 to 100,000, andpreferably from 900 to 20,000. Preferred polyalkylene glycols arepolyethylene glycol, and polypropylene glycol. Such polyalkylene glycolsare generally commercially available from a variety of sources and maybe used without further purification. Capped polyalkylene glycols whereone or more of the terminal hydrogens are replaced with a hydrocarbylgroup may also be suitably used. Examples of suitable polyalkyleneglycols are those of the formula R—O—(CXYCX′Y′O)_(n)R′ where R and R′are independently chosen from H, (C₂₋₂₀)alkyl group and (C₆₋₂₀) arylgroup; each of X, Y, X′ and Y′ is independently selected from hydrogen,alkyl such as methyl, ethyl or propyl, aryl such as phenyl, or aralkylsuch as benzyl; and n is an integer from 5 to 100,000. Typically, one ormore of X, Y, X′ and Y′ is hydrogen.

Suitable EO/PO copolymers generally have a weight ratio of EO:PO of from10:90 to 90:10, and preferably from 10:90 to 80:20. Such EO/POcopolymers preferably have an average molecular weight of from 1000 to15,000. Preferred EO/PO copolymers are block copolymers having thestructure of EO/PO/EO or PO/EO/PO. Such EO/PO copolymers are availablefrom a variety of sources, such as those available from BASF under thePLURONIC brand.

Suitable alkylene oxide condensation products of an organic compoundhaving at least one hydroxy group and 20 carbon atoms or less includethose having an aliphatic hydrocarbon of from one to seven carbon atoms,an unsubstituted aromatic compound or an alkylated aromatic compoundhaving six carbons or less in the alkyl moiety, such as those disclosedin U.S. Pat. No. 5,174,887. The aliphatic alcohols may be saturated orunsaturated. Suitable aromatic compounds are those having up to twoaromatic rings. The aromatic alcohols have up to 20 carbon atoms priorto derivatization with ethylene oxide. Such aliphatic and aromaticalcohols may be further substituted, such as with sulfate or sulfonategroups. Such suitable alkylene oxide compounds include, but are notlimited to: ethoxylated polystyrenated phenol having 12 moles of EO,ethoxylated butanol having 5 moles of EO, ethoxylated butanol having 16moles of EO, ethoxylated butanol having 8 moles of EO, ethoxylatedoctanol having 12 moles of EO, ethoxylated beta-naphthol having 13 molesof EO, ethoxylated bisphenol A having 10 moles of EO, ethoxylatedsulfated bisphenol A having 30 moles of EO and ethoxylated bisphenol Ahaving 8 moles of EO.

Formulae 3 and 4 represent nonionic surfactants which aretetrafunctional polyethers derived from the addition of differentalkylene oxides to ethylenediamine:

where A and B represent different alkyleneoxy moieties, and x and yrepresent the number of repeat units of each alkylene oxide,respectively. Preferably, A and B are chosen from (C₂₋₄)alkylene oxides,and more preferably from propyleneoxide and ethyleneoxide. Thealkyleneoxy moieties in the compounds of formulae 1 and 2 may be inblock, alternating or random arrangements. The mole ratio of x:y informulae 3 and 4 is typically from 10:90 to 90:10, and preferably from10:90 to 80:20. Such tetrafunctional polyethers are generallycommercially available, such as from BASF (Ludwigshafen, Germany) underthe TETRONIC brand.

In general, the present compositions contain water. The water may bepresent in a wide range of amounts. Any type of water may be used, suchas distilled, DI or tap.

The present compositions may optionally include one or more additives,such as antioxidants, organic solvents, alloying metals, conductivityacids, complexers, and mixtures thereof. While additional grain refinersmay be used in the present plating baths, it is preferred that theplating baths comprise only the first and second grain refiners asdiscussed above.

Antioxidants may optionally be added to the present composition toassist in keeping the tin in a soluble, divalent state. It is preferredthat one or more antioxidants are used in the present compositions.Exemplary antioxidants include, but are not limited to, hydroquinone,and hydroxylated aromatic compounds, including sulfonic acid derivativesof such aromatic compounds, and preferably are: hydroquinone;methylhydroquinone; resorcinol; catechol; 1,2,3-trihydroxybenzene;1,2-dihydroxybenzene-4-sulfonic acid;1,2-dihydroxybenzene-3,5-disulfonic acid;1,4-dihydroxybenzene-2-sulfonic acid;1,4-dihydroxybenzene-2,5-disulfonic acid; and 2,4-dihyroxybenzenesulfonic acid. Such antioxidants are disclosed in U.S. Pat. No.4,871,429. Other suitable antioxidants or reducing agents include, butare not limited to, vanadium compounds, such as vanadylacetylacetonate,vanadium triacetylacetonate, vanadium halides, vanadium oxyhalides,vanadium alkoxides and vanadyl alkoxides. The concentraion of suchreducing agent is well known to those skilled in the art, but istypically in the range of from 0.1 to 10 g/L, and preferably from 1 to 5g/L. Such antioxidants are generally commercially available from avariety of sources.

Optional organic solvents may be added to the present tin electroplatingcomposition. Typical solvents useful in the present compositions arealiphatic alcohols. Preferred organic solvents are methanol, ethanol,n-propanol, iso-proponol, n-butanol, and iso-butanol. Such solvent maybe used in the present tin electroplating compositions in an amount iffrom 0.05 to 15 g/L, and preferably from 0.05 to 10 g/L.

Optionally, the present plating baths may contain one or more sources ofalloying metal ions. Suitable alloying metals include, withoutlimitation, silver, gold, copper, bismuth, indium, zinc, antimony,manganese and mixtures thereof. Preferred alloying metals are silver,copper, bismuth, indium, and mixtures thereof, and more preferablysilver. It is preferred that the present compositions are free of lead.Any bath-soluble salt of the alloying metal may suitably be used as thesource of alloying metal ions. Examples of such alloying metal saltsinclude, but are not limited to: metal oxides; metal halides; metalfluoroborate; metal sulfates; metal alkanesulfonates such as metalmethanesulfonate, metal ethanesulfonate and metal propanesulfonate;metal arylsulfonates such as metal phenylsulfonate, metaltoluenesulfonate, and metal phenolsulfonate; metal carboxylates such asmetal gluconate and metal acetate; and the like. Preferred alloyingmetal salts are metal sulfates; metal alkanesulfonates; and metalarylsulfonates. When one alloying metal is added to the presentcompositions, a binary alloy deposit is achieved. When 2, 3 or moredifferent alloying metals are added to the present compositions,tertiary, quaternary or higher order alloy deposits are achieved. Theamount of such alloying metal used in the present compositions willdepend upon the particular tin-alloy desired. The selection of suchamounts of alloying metals is within the ability of those skilled in theart. It will be appreciated by those skilled in the art that whencertain alloying metals, such as silver, are used, an additionalcomplexing agent may be required. Such complexing agents (or complexers)are well-known in the art and may be used in any suitable amount.

Conductivity acids may optionally be added to the present compositions.Such conductivity acids include, but are not limited to, boric acid,alkanoic acids, hydroxyalkanoic acids, and salts of these acids to theextent they are water-soluble. Preferred are formic acid, acetic acid,oxalic acid, citric acid, malic acid, tartaric acid, gluconic acid,glucaric acid, glucuronic acid, and salts of these acids. When used,such conductivity acids and salts are employed in conventional amounts.

The electroplating compositions of the invention may be prepared by anysuitable method known in the art. Typically, they are prepared by addingthe acid electrolyte to a vessel, followed by one or more tin compounds,one or more surfactants, one or more grain refiners, optionally waterand any other optional components. Other orders of addition of thecomponents of the compositions may be used. Once the composition isprepared, any undesired material is removed, such as by filtration, andthen water is added to adjust the final volume of the composition. Thecomposition may be agitated by any known means, such as stirring,pumping, sparging or jetting the composition, for increased depositionspeed.

Plating baths of the invention are acidic, that is, they have a pH of<7. Typically, the pH of the present plating baths is from −1 to <7,preferably from −1 to 6.5, more preferably from −1 to 5, and yet morepreferably from −1 to 2.

The present electroplating compositions are suitable for depositing atin-containing layer, which may be a pure tin layer or a tin-alloylayer. Exemplary tin-alloy layers include, without limitation,tin-silver, tin-silver-copper, tin-silver-copper-antimony,tin-silver-copper-manganese, tin-silver-bismuth, tin-silver-indium,tin-silver-zinc-copper, and tin-silver-indium-bismuth. Preferably, thepresent electroplating compositions deposit pure tin, tin-silver,tin-silver-copper, tin-silver-bismuth, tin-silver-indium, andtin-silver-indium-bismuth, and more preferably pure tin, tin-silver ortin-silver-copper. Alloys deposited from the present electroplating bathcontain an amount of tin ranging from 0.01 to 99.99 wt %, and an amountof one or more alloying metals ranging from 99.99 to 0.01 wt %, based onthe weight of the alloy, as measured by either atomic adsorptionspectroscopy (AAS), X-ray fluorescence (XRF), inductively coupled plasma(ICP) or differential scanning calorimetry (DSC). Preferably, thetin-silver alloys deposited using the present invention contain from 75to 99.99 wt % tin and 0.01 to 10 wt % of silver and any other alloyingmetal. More preferably, the tin-silver alloy deposits contain from 95 to99.9 wt % tin and 0.1 to 5 wt % of silver and any other alloying metal.Tin-silver alloy is the preferred tin-alloy deposit, and preferablycontains from 90 to 99.9 wt % tin and from 10 to 0.1 wt % silver. Morepreferably, the tin-silver alloy deposits contain from 95 to 99.9 wt %tin and from 5 to 0.1 wt % silver. For many applications, the eutecticcomposition of an alloy may be used. Alloys deposited according to thepresent invention are substantially free of lead, that is, theycontain≦1 wt % lead, more preferably ≦0.5 wt %, and yet more preferably≦0.2 wt %, and still more preferably are free of lead.

The plating compositions of the present invention are useful in variousplating methods where a tin-containing layer is desired, andparticularly for depositing a tin-containing solder layer on asemiconductor wafer comprising a plurality of conductive bondingfeatures. Plating methods include, but are not limited to, horizontal orvertical wafer plating, barrel plating, rack plating, high speed platingsuch as reel-to-reel and jet plating, and rackless plating, andpreferably horizontal or vertical wafer plating. A wide variety ofsubstrates may be plated with a tin-containing deposit according to thepresent invention. Substrates to be plated are conductive and maycomprise copper, copper alloys, nickel, nickel alloys, nickel-ironcontaining materials. Such substrates may be in the form of electroniccomponents such as lead frames, connectors, chip capacitors, chipresistors, and semiconductor packages; plastics such as circuit boards;and semiconductor wafers; and preferably are semiconductor wafers.Accordingly, the present invention also provides a method of depositinga tin-containing layer on a semiconductor wafer comprising: providing asemiconductor wafer comprising a plurality of conductive bondingfeatures; contacting the semiconductor wafer with the compositiondescribed above; and applying sufficient current density to deposit atin-containing layer on the conductive bonding features. Preferably, thebonding features comprise copper, which may be in the form of a purecopper layer, a copper alloy layer, or any interconnect structurecomprising copper. Copper pillars are one preferred conductive bondingfeature. Optionally, the copper pillars may comprise a top metal layer,such as a nickel layer. When the conductive bonding features have a topmetal layer, then the pure tin solder layer is deposited on the topmetal layer of the bonding feature. Conductive bonding features, such asbonding pads, copper pillars, and the like, are well-known in the art,such as described in U.S. Pat. No. 7,781,325, and in U.S. Pat. Pub. Nos.2008/0054459, 2008/0296761, and 2006/0094226.

As used herein, the term “semiconductor wafer” is intended to encompass“an electronic device substrate,” “a semiconductor substrate,” “asemiconductor device,” and various packages for various levels ofinterconnection, including a single-chip wafer, multiple-chip wafer,packages for various levels, or other assemblies requiring solderconnections. Particularly suitable substrates are patterned wafers, suchas patterned silicon wafers, patterned sapphire wafers and patternedgallium-arsenide wafers. Such wafers may be any suitable size. Preferredwafer diameters are 200 mm to 300 mm, although wafers having smaller andlarger diameters may be suitably employed according to the presentinvention. As used herein, the term “semiconductive substrates” includesany substrate having one or more semiconductor layers or structureswhich include active or operable portions of semiconductor devices. Theterm “semiconductor substrate” is defined to mean any constructioncomprising semiconductive material, including but not limited to bulksemiconductive material such as a semiconductive wafer, either alone orin assemblies comprising other materials thereon, and semiconductivematerial layers, either alone or in assemblies comprising othermaterials. A semiconductor device refers to a semiconductor substrateupon which at least one microelectronic device has been or is beingbatch fabricated.

A substrate is plated with a tin-containing layer by contacting thesubstrate with the present compositions and applying a current densityfor a period of time to deposit the tin-containing layer on thesubstrate. Such contact is may be by placing the substrate to be platedin the plating bath composition, or pumping the plating bath compositiononto the substrate. The substrate is conductive and is the cathode. Theplating bath contains an anode, which may be soluble or insoluble.Potential is typically applied to the cathode. Sufficient currentdensity is applied and plating performed for a period of time sufficientto deposit a tin-containing layer having a desired thickness on thesubstrate. Semiconductor wafers comprising a plurality of bondingfeatures are electroplated by contacting the wafer with a plating bathof the present invention and applying a current density for a period oftime to deposit a tin-containing layer on the plurality of bondingfeatures. The semiconductor wafer functions as the cathode.

The particular current density used to deposit the tin-containing layerdepends on the particular plating method, the substrate to be plated,and whether a pure tin or tin-alloy layer is to be deposited. Suitablecurrent density is from 0.1 to 200 A/dm². Preferably the current densityis from 0.5 to 100 A/dm², more preferably from 0.5 to 30 A/dm², evenmore preferably from 0.5 to 20 A/dm², and most preferably from 2 to 20A/dm². Other current densities may be useful depending upon theparticular bonding feature to be plated, as well as other considerationsknown to those skilled in the art. Such current density choice is withinthe abilities of those skilled in the art.

Tin-containing layers may be deposited at a temperature of 10° C. orhigher, preferably in the range of from 10 to 65° C., and morepreferably from 15 to 40° C. In general, the longer the time thesubstrate is plated the thicker the deposit while the shorter the timethe thinner the deposit for a given temperature and current density.Thus, the length of time a substrate remains in a plating compositionmay be used to control the thickness of the resulting tin-containingdeposit. In general, metal deposition rates may be as high as 15 μm/min.Typically, deposition rates may range from 0.5 to 15 μm/min, andpreferably from 1 to 10 μm/min.

While the present electrolyte compositions may be used for a variety ofapplications as described above, an exemplary application is forinterconnect bump (solder bump) formation for wafer-level-packaging.This method involves providing a semiconductor die (wafer die) having aplurality of conductive bonding features (such as interconnect bumppads), forming a seed layer over the bonding features, depositing atin-containing interconnect bump layer over the bonding features bycontacting the semiconductor die with the present electroplatingcomposition and passing a current through the electroplating compositionto deposit the tin-containing interconnect bump layer on the substrate,and then reflowing the interconnect bump layer to form a solder bump.The conductive interconnect bump pad may be one or more layers of ametal, composite metal or metal alloy typically formed by physical vapordeposition (PVD) such as sputtering. Typical conductive bonding featurescomprise, without limitation, aluminum, copper, titanium nitride, andalloys thereof.

A passivation layer is formed over the bonding features and openingsextending to the bonding features are formed therein by an etchingprocess, typically by dry etching. The passivation layer is typically aninsulating material, for example, silicon nitride, silicon oxynitride,or a silicon oxide, such as phosphosilicate glass. Such materials may bedeposited by chemical vapor deposition (CVD) processes, such as plasmaenhanced CVD. An under bump metallization (UBM) structure formedtypically of a plurality of metal or metal alloy layers, is depositedover the device. The UBM acts as an adhesive layer and electricalcontact base (seed layer) for an interconnect bump to be formed. Thelayers forming the UBM structure may be deposited by PVD, such assputtering or evaporation, or CVD processes. Without limitations, theUBM structure may be, for example, a composite structure including inorder, a bottom chrome layer, a copper layer, and an upper tin layer.Nickel is one metal used in UBM applications.

In general, a photoresist layer is applied to a semiconductor wafer,followed by standard photolithographic exposure and developmenttechniques to form a patterned photoresist layer (or plating mask)having openings or vias therein (plating vias). The dimensions of theplating mask (thickness of the plating mask and the size of the openingsin the pattern) defines the size and location of the tin-silver layerdeposited over the I/O pad and UBM. The diameters of such depositstypically range from 5 to 300 μm, preferably from 10 to 150 μm. Theheight of such deposits typically range from 10 to 150 μm, preferablyfrom 15 to 150 μm, and more preferably from 20 to 80 μm. Suitablephotoresist materials are commercially available (such as from DowElectronic Materials, Marlborough, Mass., USA) and are well-known in theart.

The interconnect bump material is deposited on the device by anelectroplating process using the above-described electroplatingcompositions. Interconnect bump materials include, for example, pure tinor any suitable tin alloys. Exemplary tin alloys are those describedabove. It may be desired to use such alloys at their eutecticcompositions, or at any other suitable compositions. The bump materialis electrodeposited in the areas defined by the plating via. For thispurpose, a horizontal or vertical wafer plating system, for example, afountain plating system, is typically used with a direct current orpulse-plating technique. In the plating process, the interconnect bumpmaterial completely fills the via extending above and on a portion ofthe top surface of the plating mask, resulting in a mushroom-shapedmetal deposit. This ensures that a sufficient volume of interconnectbump material is deposited to achieve the desired ball size afterreflow. In the via plating process, the photoresist has a sufficientthickness such that the appropriate volume of interconnect bump materialis contained within the plating mask via. A layer of copper or nickelmay be electrodeposited in the plating via prior to plating theinterconnect bump material. Such a layer may act as a wettablefoundation to the interconnect bump upon reflow.

Following deposition of the interconnect bump material the plating maskis stripped using an appropriate solvent or other remover. Such removersare well known in the art. The UBM structure is then selectively etchedusing known techniques, removing all metal from the field area aroundand between interconnect bumps.

Alternatively, after plating vias are formed in the photoresist layer, ametal interconnect structure, such as a pillar, that is free of tin, maybe deposited on the bonding features. Copper pillars are conventional.Typically, such metal deposition will stop before the plating via iscompletely filled. Such interconnect structures, such as copper pillars,may then be capped by a tin-containing layer by an electroplatingprocess using the above-described electroplating compositions. Thetin-containing layer is electrodeposited in the areas defined by theplating via. For this purpose, a horizontal or vertical wafer platingsystem, for example, a fountain plating system, is typically used with adirect current or pulse-plating technique. A layer of a top metal, suchas nickel, may be electrodeposited in the plating via on top of thecopper pillar prior to plating the tin-containing layer. Such a topmetal layer may act as a wettable foundation to pure tin solder layer,and/or provide a barrier layer. The height of such tin-containing layermay range from 20 to 50 μm although other heights may be suitable, andhas a diameter substantially equal to that of the interconnect structureon which it is deposited. Following deposition of the tin-containingsolder layer, the plating mask is stripped using an appropriate solventor other remover. Such removers are well known in the art. The UBMstructure is then selectively etched using known techniques, removingall metal from the field area around and between interconnect bumps.

The wafer is then optionally fluxed and is heated in a reflow oven to atemperature at which the tin-containing solder layer melts and flowsinto a truncated substantially spherical shape. Heating techniques areknown in the art, and include, for example, infrared, conduction, andconvection techniques, and combinations thereof. The reflowedinterconnect bump is generally coextensive with the edges of the UBMstructure. The heat treatment step may be conducted in an inert gasatmosphere or in air, with the particular process temperature and timebeing dependent upon the particular composition of the interconnect bumpmaterial.

Tin-containing solders electodeposited from the present compositions aresubstantially free of voids when deposited (or plated), preferably thesesolders are substantially free of voids after one reflow cycle andpreferably after repeated reflow cycles, such as after 3 reflow cycles,and more preferably after 5 reflow cycles. A suitable reflow cycle usesa Falcon 8500 tool from Sikama International, Inc., having 5 heating and2 cooling zones, using temperatures of 140/190/230/230/260° C., with a30 second dwell time, and a conveyor rate of ca. 100 cm/min. and anitrogen flow rate of 40 cubic feet/hour. Alpha 100-40 flux (CooksonElectronics, Jersey City, N.J., USA) is a suitable flux used in thisreflow process. As used herein, the term “voids” refers to bothinterfacial voids and voids within the bulk tin-containing layer. By“substantially free of voids” is meant that no void of a diameter largerthan 3 μm, preferably 2 μm, and more preferably 1 μm, is visible using aCougar microfocus X-ray system (YXLON International GmbH, Hamburg,Germany).

The present electroplating compositions also provide tin-containingsolder deposits having a relatively smooth surface as plated, that is,the surface as plated has a mean surface roughness (R_(a)) of ≦200 nm,preferably ≦150 nm and even more preferably ≦100 nm, as measured usingan optical profilometer (Leica DCM3D, Leica Microsystems GmbH, Wetzlar,Germany).

EXAMPLE 1

An electroplating composition for depositing pure tin was prepared bycombining 75 g/L tin (from tin methanesulfonate), 140 g/Lmethanesulfuonic acid, 8.8 g/L of an ethoxylated beta-naphthol nonionicsurfactant, 9.7 g/L of an ethoxylated bisphenol A nonionic surfactant,0.0166 g/L of benzylidene acetone as the first grain refiner, 0.1 g/L ofmethacrylic acid as the second grain refiner, 1.5 g/L of alcoholsolvent, 1 g/L of a commercial antioxidant, and DI water (balance). ThepH of the composition was <1.

EXAMPLE 2

The composition of Example 1 was used to deposit pure tin on aconductive bonding feature. Wafer segments of 4 cm×4 cm with photoresistpatterned vias of 75 μm (diameter)×50 μm (depth) and pre-formed copperpillars of 37 μm height were immersed in a plating cell containing thecomposition of Example 1 and plated with a pure tin layer. Samples wereplated in the bath at 8 A/dm². The temperature of the bath was at 25° C.An insoluble platinized titanium electrode was used as the anode, andthe wafer segment was the cathode. Electroplating was done until amushroom shaped tin cap of 23 μm height was plated on top of copperpillars. Morphology of the resulting tin layer was inspected with aHitachi S2460™ scanning electron microscope. The pure tin deposits wereuniform, smooth, compact, and free of nodules.

EXAMPLE 3

An electroplating composition for depositing a tin-silver alloy wasprepared by combining 75 g/L tin (from tin methanesulfonate), 0.65 g/Lsilver (from silver methanesulfonate), 104 g/L methanesulfuonic acid, 5g/L of a tetrafunctional block copolymer derived from the sequentialaddition of E0 and PO to ethylenediamine nonionic surfactant (EO:PO ofapproximately 40:60, TETRONIC 90R4), 0.0166 g/L of benzylidene acetoneas the first grain refiner, 0.1 g/L of methacrylic acid as the secondgrain refiner, 0.6 g/L of a dithiaalkyldiol as a first silver complexingagent, 3.1 g/L of a mercaptotetrazole derivative as a second silvercomplexing agent, 1.5 g/L of alcohol solvent, 1 g/L of a commercialantioxidant, and DI water (balance). The pH of the composition was <1.

Wafer segments of 4 cm×4 cm with photoresist patterned vias of 75 μm(diameter)×72 μm (depth) and a copper seed layer were immersed in aplating cell containing the tin-silver electroplating composition andplated with tin-silver bumps. Samples were plated in the bath at 8A/dm². The temperature of the bath was at 25° C. An insoluble platinizedtitanium electrode was used as the anode. Electroplating was done untila bump of 60 μm was plated. The resulting tin-silver solder bump depositwas flat and had a smooth morphology with a mean surface roughness of150 nm as measured using a Leica DCM3D optical profilometer. Thetin-silver solder deposit was reflowed using a Falcon 8500 tool (SikamaInternational, Inc.) having 5 heating and 2 cooling zones, usingtemperatures of 140/190/230/230/260° C., with a 30 second dwell time,and a conveyor rate of ca. 100 cm/min. and a nitrogen flow rate of 40cubic feet/hour. The solder deposits were fluxed with Alpha 100-40 flux.The reflowed tin-silver deposit was evaluated using a Cougar microfocusX-ray system (YXLON International GmbH, Hamburg, Germany) and was foundto be free of voids.

EXAMPLE 4

Various tin-alloy electroplating compositions are prepared by repeatingthe procedure of Example 3 and replacing the source of silver ions withthe sources of metal ions listed in Table 1.

TABLE 1 Sample Alloy Source of Alloying Metal Ions 4-1 Sn—Bi Bismuthmethansulfonate (10 g/L Bi) 4-2 Sn—Cu Copper methanesulfonate (0.3 g/LCu) 4-3 Sn—Ag—Cu Silver methanesulfonate (0.6 g/L Ag) Coppermethanesulfonate (0.2 g/L Cu) 4-4 Sn—In Indium methanesulfonate (50 g/LIn)

EXAMPLE 5

The electroplating composition of Examples 1 and 3 are repeated exceptthat the grain refiners shown in Table 2 are used.

TABLE 2 Sample First Grain Refiner Second Grain Refiner 5-1 PicolinicAcid (0.015 g/L) Methacrylic acid (0.1 g/L) 5-2 Cinnamaldehyde (0.022g/L) Methacrylic acid (0.15 g/L) 5-3 Benzylidene Acetone (0.013 g/L)Crotonic acid (0.25 g/L) 5-4 Cinnamaldehyde (0.008 g/L) Methacrolein(0.35 g/L) 5-5 Pyridinycarboxaldehyde Methacrylic acid (0.35 g/L) (0.005g/L) 5-6 Cinnamic acid (0.025 g/L) Acrolein (0.08 g/L) 5-7 BenzylideneAcetone (0.022 g/L) Methacrolein (0.13 g/L) 5-8 Pyridinedicarboxylicacid Crotonaldehyde (0.32 g/L) (0.0075 g/L) 5-9 Benzylidene Acetone(0.005 g/L) Acrylic acid (0.4 g/L)

COMPARATIVE EXAMPLE 1

The procedure of Example 3 was repeated except that the amount of thefirst grain refiner, benzylidene acetone, was 0.166 g/L. The resultingtin-silver bump deposit was significantly concave, and skewed to oneside of the bump. Evaluation of the deposit by microfocus X-ray showedthe formation of voids (>3 μm) after one reflow of the deposit.

COMPARATIVE EXAMPLE 2

The procedure of Example 3 was repeated except that no first grainrefiner was used. The resulting tin-silver bump deposit was unacceptablyrough, having a mean surface roughness (Ra) over 800 nm as measured by aLeica DCM3D optical profilometer.

What is claimed is:
 1. An electroplating composition comprising: asource of tin ions; an acid electrolyte; 0.0001 to 0.045 g/L of a firstgrain refiner; 0.005 to 0.75 g/L of an α,β-unsaturated aliphaticcarbonyl compound as a second grain refiner; a nonionic surfactant; andwater; wherein the first grain refiner is chosen from a compound offormula (1) or (2)

wherein each R¹ is independently (C₁₋₆)alkyl, (C₁₋₆)alkoxy, hydroxy, orhalo; R² and R³ are independently chosen from H and (C₁₋₆)alkyl; R⁴ isH, OH, (C₁₋₆)alkyl or O(C₁₋₆)alkyl; m is an integer from 0 to 2; each R⁵is independently (C₁₋₆)alkyl; each R⁶ is independently chosen from H,OH, (C₁₋₆)alkyl, or O(C₁₋₆)alkyl; n is 1 or 2; and p is 0, 1 or
 2. 2.The composition of claim 1 further comprising a source of alloying metalions.
 3. The composition of claim 1 wherein the α,β-unsaturatedaliphatic carbonyl compound is chosen from α,β-unsaturated carboxylicacids, α,β-unsaturated carboxylic acid esters, α,β-unsaturated amides,and α,β-unsaturated aldehydes.
 4. The composition of claim 3 wherein theα,β-unsaturated aliphatic carbonyl compound is chosen from (meth)acrylicacid, crotonic acid, (C₁₋₆)alkyl (meth)acrylate, (meth)acrylamide,(C₁-C₆)alkyl crotonate, crotonamide, crotonaldehyde, or mixturesthereof.
 5. The composition of claim 1 wherein the first grain refineris chosen from cinnamic acid, cinnamaldehyde, benzylidene acetone,picolinic acid, pyridinedicarboxylic acid, pyridinecarboxaldehyde,pyridinedicarboxaldehyde, or mixtures thereof.
 6. The composition ofclaim 1 wherein the first grain refiner is present in an amount of 0.001to 0.04 g/L.
 7. The composition of claim 1 wherein the nonionicsurfactant comprises an alkoxylated amine moiety.
 8. A method ofdepositing a tin-containing layer on a semiconductor substratecomprising: providing a semiconductor wafer comprising a plurality ofconductive bonding features; contacting the semiconductor wafer with thecomposition of claim 1; and applying sufficient current density todeposit a tin-containing layer on the conductive bonding features. 9.The method of claim 8 wherein the bonding features comprise copper. 10.The method of claim 8 wherein the tin-containing layer on the bondingfeatures is substantially free of voids as plated and after one reflowcycle.